26th Texas A&MInternational Pump Users Symposium
March, 2010
Evaluating and CorrectingSubsynchronous Vibration
In Vertical Pumps
Malcolm E. LeaderKelly J ConnerJamie D. Lucas
Case Study #8
Case Study Overview
• New Vertical Pumps• Liquid Sulfur Service• Operating Speed – 3,575 RPM• Primary Vibration Component at 1,750 CPM• Shaft Whirling or Whipping Suspected• Structural Resonance near ½ X• Solution
Pump Layout
164Inches
Pump System6 Vertical Pumps in 3 Separate Sumps
Single Stage 81 GPM 219 Ft Head 40 HP
Molten Sulfur at 300 °F
Pump and Shaft are Steam Jacketed
4 Radial Bearings/Bushings
Top Bearing is also Thrust Bearing
Line Shaft Bushings are Carbon Graphite
History• Hard “Sulcrete” Buildup in Sumps
• New Pump Mounting System Devised
• Initial Base Impact Test Showed Structural Natural Frequency in Excess of 5,000 CPM
• Initial Vibration Readings:
First Installation had 0.2 IPS at 1,710 CPM
Second Installation had 0.65 IPS at 1,750 CPM
Vibration Measurements• High Vibration on All Units
• Virtually no 1X (Running Speed) Vibration
• Most Vibration near ½X Running Speed
Measured Spectrum on Motor
History• Installed Impact Test Results on Motor
Bump TestLocation
East - WestDirection
North - SouthDirection
East Pump
West Pump
1,734 CPM
1,657 CPM 1,887 CPM
1,887 CPM
History• Clearly There was a System Natural Frequency near Half of Operating Speed
• What was the “Forcing Function”?
• Sulfur “whirl” Considered
Initial Attempts• Reduced Weight of Coupling
No Significant Effect
• Possibly Stiffen Structure?
Probably would be Ineffective – Not Done
• Rotordynamics Analysis
Selected Approach
Rotordynamics Analysis• Gather Data from Disassembled Unit
• Dimensions and Weights of all Rotating Parts
• Evaluate Bushing Dimensions
• Determine Properties of Liquid Sulfur
• Translate into Finite Element Model
• Match Model to Observed Vibrations
• Design New Components to Fix Problem
Actual System Compared to Model
164Inches
FEAModel
Imbalance
Imbalance
ThrustBearing
PumpBearing
TopBushing
LowerBushing
2.5 InchDiameter
Shaft
Center Two Rotating Sleeves
OriginalDesign
ModifiedDesign
Properties of Molten Sulfur• Unusual Material
• Room Temperature Sulfur is a Crystalline Solid
• Fully Liquid at 235°F
• Forms Long-Chain Molecules – Gamma Sulfur
• Highly Non-Linear Viscosity
7.8 cP at 300°F 93,000 cP at 350°F
• Sump Temperature Control Very Important
Crystalline Solid Sulfur
Sulfur Viscosity v. Temperature
Original Bushing Design
Diametral Clearance
16 Mils
= 0
First Critical SpeedMode Shape
Calculated Frequency = 1,770 CPMWith Original Upper and Lower Bushings
Original Unbalance Response
Calculated ResponseAt Lower Bushing
1,785 CPM
Stability with Original Bushings
Stability with Original Bushings
Non-LinearTime Transient
AnalysisCalculated Spectrum
At Lower Bushing
Evaluation• Shaft Critical Speed at ½ Operating Speed
• Structural Resonance at ½ Operating Speed
• Inherently Unstable Plain Circular Bushings
• Need To Control The Rotor Vibration
• Center Two Bushings Logical Items to Revise
Solution Goals• Design New Upper and Lower Bushings
Increase Direct Stiffness
Raise Critical Speed above Operating Speed
Eliminate Instability
Assure Low Operating Speed Vibration
Increase Reliability and Reduce Costs
Optimized Bushing Design
Diametral Clearance
9 Mils
Optimized Bushing Pressure Profile
Peak Pressure27 PSI
Bushing Stiffness and Damping Coefficient Comparison at
3,575 RPMBushing
TypePrincipal Stiffness,
LB/INPrincipal Damping,
LB-SEC/INCross-Coupled
Stiffness, LB/INHorsepower
Loss
Plain Bushing
1,830 407 -69,400 0.65
3-Lobe Bushing
49,100 323 -20,700 0.38
(26.8 Times Stiffer)
New Design Unbalance Response
Calculated ResponseAt Lower Bushing
Time TransientAnalysis
Stability with New Design Bushings
Time TransientAnalysis
Calculated SpectrumAt Lower Bushing
Final Result
Measured Spectrum on Motor
Conclusion• Vibration Problem Eliminated
Replaced 2 Plain Bushings with Profile Design
Inexpensive
Available Locally
Eliminated Critical Speed
No Structural Modifications Necessary
Reliability Increased and Costs Reduced